At oil and gas well sites, particularly where drilling is conducted in shale formations, there is an array of equipment, as for example tank batteries to collect crude oil and/or distillates from the oil and gas wells, as well as separators to separate gas/water from hydrocarbons. Generally speaking, tank batteries are a source of low pressure flare gas while separators are a source of high pressure flare gas. In either event, the gases cannot be allowed to accumulate as the pressure build up could create hazards to humans as well as potential damage to equipment. Nor can they be vented to atmosphere for environmental reasons. To alleviate this problem, these gases, both high and low pressure, are vented from the equipment and flared using a suitable flare gas assembly.
Low pressure gases from tank batteries, i.e., tanks that hold product (oil) for truck loading, present a challenge. Generally speaking, tank batteries are at atmospheric pressure and venting allows the product to easily flow in and out. However, the low pressure gas vented cannot be allowed to escape to the atmosphere least environmental regulations be violated. From a practical perspective, the only way to prevent these low pressure hydrocarbon emissions from escaping to the atmosphere is by flaring.
A typical tank battery is equipped with relief valves, such as Kimray valves well known to those skilled in the art, which relieve pressure from the tank when it exceeds about 4 to 5 ounces, although the relief valve can be set to vent at higher pressures, e.g., 10 ounces. The gas relieved from the pressure relief valve must, as discussed above, be flared. Flaring of low pressure tank battery gas can pose a problem not encountered in flaring of high pressure flare gas. High pressure gases generally have sufficient kinetic energy and do not require assist to burn smokelessly. However, because of its low pressure and insufficient kinetic energy, vented gas from tank batteries is normally flared using air assist flares. Typically, the air assist comes from a centrifugal or axial blower mounted at the bottom or side of the flare stack and a typical prior art flare handling low pressure tank batter emissions may have two 150 horsepower air blowers.
It is known that a properly operated low pressure air flare can achieve well over 98% destruction and removal efficiency (DRE) wherein DRE is the percent removal of hydrocarbon from the flare vent gas, provided that the air/hydrocarbon ratio is kept within a certain range. Thus, too much air can blow out the flame creating hydrocarbon emission detectible on Fourier Transfer Infrared (FTIR) cameras. In an attempt to overcome this problem, and maintain the air/hydrocarbon ratio in the desired range, prior art air flares handling low pressure flare gas, e.g., from tank batteries, typically employ blowers driven by electric motors with variable frequency (or variable speed) drives (VFD). These set ups also require additional, expensive equipment such as flow meters and process controllers, e.g., programmable logic controllers (PLCs) for efficient operation.